The present invention is directed to a method and apparatus for sampling and holding an analog input voltage which eliminates offset voltage error and more specifically to such a circuit which is especially useful in a 12 bit recirculating Gray code analog to digital converter.
An analog to digital converter typically includes, especially when it is of the monolithic type, an R-2R resistive ladder network which requires laser trimming. When a 12 bit converter is required there are serious problems with temperature coefficients and lack of automatic zeroing. Where a recirculating converter incorporating a sample and hold circuit is used, the foregoing problem is eliminated. However, problems of offset voltage and common mode voltage errors are then introduced. Manufacturers have attacked this problem by use of transformers which while tending to eliminate offset errors introduce accompanying mechanical and electrical problems. In addition, such technique requires a zeroing cycle which adds to the time required for conversion. Other manufacturers have produced converters where the offset adjustment and/or temperature compensation networks were added along with perhaps a zeroing mode of operation.
In a 12 bit converter if, for example, a 2,000 millivolt analog input voltage is being measured, the least significant bit will indicate one-half millivolt and this is thus the required accuracy. Most offset errors begin at 1 to 2 millivolts and increase due to common mode voltages.